just get the cheapest stuff you can find at walmart or any discount clothing store or better yet thrift store...Unless you want to actually look nice while your wearing them, for that you may have to dig a little deeper.
What do you do that gets the clothes so dirty? I would suggest keeping a set of clothes that is specifically for doing dirty work and just let them get stained the fuck up. maybe you have a shirt that u liked but it got stained? Instead of throwing it away keep it and only wear it when you are doing dirty stuff.